Heretofore, a glass plate (glass substrate) used for various displays, particularly a glass substrate used for a glass plate on which surface a metal or oxide thin film is formed, has been required to have the following characteristics:
(1) Not substantially containing alkali metal ions; because in the case where the glass contains an alkali metal oxide, alkali metal ions diffuse in the thin film, resulting in deterioration of film characteristics.
(2) Having a high strain point so that deformation of a glass plate and shrinkage (thermal shrinkage) due to structure stabilization of the glass can be minimized when exposed to high temperature in a thin film formation step.
(3) Having sufficient chemical durability against various chemicals used in semiconductor formation; in particular, having durability against buffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid and ammonium fluoride) for etching SiOx or SiNx, against a chemical solution containing hydrochloric acid used for etching of ITO, against various acids (nitric acid, sulfuric acid, etc.) used for etching of an metal electrode, and against an alkaline of a resist removing liquid.(4) Having no defects (bubbles, striae, inclusions, pits, flaws, etc.) in the inside and on the surface.
In addition to the above requirements, the recent situations are as follows.
(5) Reduction in weight of a display is required, and the glass itself is also required to be a glass having a small density.
(6) Reduction in weight of a display is required, and a decrease in thickness and improvement of Young's modulus of the glass plate is desired.
(7) In addition to conventional amorphous silicon (a-Si) type liquid crystal displays, polycrystal silicon (p-Si) type liquid crystal displays requiring a slightly high heat treatment temperature have come to be produced (a-Si: about 350° C.→p-Si: 350 to 550° C.) and thus, thermal resistance is required.(8) In order to improve productivity and increase thermal shock resistance by increasing the rate of rising and falling temperature in heat treatment for preparation of a liquid crystal display, a glass having a small average thermal expansion coefficient is required.
On the other hand, small and medium-sized displays for mobile as typified by a smartphone have progressed in high definition and thus, the above demands have become more and more strict.
For example, there are non-alkali glasses proposed in the Patent Documents 1 to 3.